This invention uses ultrasonic waves or radar microwaves for detecting the dynamic fluidized bed level in a fluidized bed polymerization reactor.
Many commonly used fluidized-bed reactors for olefin polymerization contain a dense xe2x80x9cbedxe2x80x9d in the reactor. The bed can be a gas-solid dense fluidized bed with or without the xe2x80x9ccondensing-modexe2x80x9d operationxe2x80x94that is, with or without the injection of liquid into the bed. Gas-phase polymerization reactors can be designed for either one-pass gas or for recycling of the gas. In either case, the location of bed level (or material level) in the reactor can be an important factor in the calculation and/or control of various limits of the process conducted in the reactor.
Usually in the past, the bed level of a fluidized bed in a fluidized-bed polyolefin reactor has been monitored by reading pressure differences among taps installed on the reactor wall. However, the taps can be easily blocked by particles, which causes a false bed-level reading. Also the bed-level measurement through pressure taps can be negatively affected by channeling and/or local defluidization in the reactor, particularly in some operations prone to local channeling, such as in the condensing mode and/or where there are sticky polymer particles. On occasion, the reactor is shut down due to problems in reading or controlling the bed level, even though the pressure taps may be blown free periodically.
Other commonly available methods for material level detection include invasive detectors which employ a simple probe inserted into the bed, and non-invasive radioactive detectors installed on the side of a reactor. Invasive probe detectors are vulnerable to fouling on the probe inserted into the bed. Radioactive detectors usually do not offer good accuracy and provoke environmental and safety concerns.
Fluidized beds, particularly the fluidized beds for large olefin polymerization reactors, present special problems for bed level measurement. Even in fluidized beds containing no liquid, bubbles are clearly present and discernable. The bubbles may vary in diameter from 0.05 meter to 4.0 meters, depending on the reactor""s size and operating conditions. Bubbles are devoid of suspended particles, but significant quantities of particles are known to be ejected into the freeboard by bubbles in at least two waysxe2x80x94by ejection at the top surface of the bubble when it reaches the bed surface, and by expulsion from the wake of the bubble after it erupts through the surface of the bed. See xe2x80x9cPrinciples of Gas-Solid Flowsxe2x80x9d by Liang-shih Fan and Chao Zhu, Cambridge Press (1988), page 401. Commonly the freeboard above the bed level will contain relatively coarse particles in its lower regions and finer ones higher up. In olefin polymerization, different types of polymers have different patterns of suspended density throughout the bed and above it. Because of the turmoil on the surface of the bed caused to a large extent by bursting bubbles, the bed level in a fluidized bed has been difficult to measure. We call such a bed level one characterized by substantially continuous bursting of bubbles of 0.05 to 4 meters in diameterxe2x80x94a dynamic bed level, or a dynamic fluidized-bed level.
U.S. Pat. No. 4,993,264 describes a method to use passive acoustics technology to monitor fluidized-bed level. A number of accelerometers were used on the side wall of a fluidized bed (not a polymerization reactor) to measure the wall vibration. By comparing the RMS acceleration from the accelerometers, the bed level can be defined. This technique requires several accelerometers and is more complicated than a single sensor or a single pair of sensors located on the top of the vessel. It is not practical for measuring dynamic bed level in a fluidized-bed polymerization reactor, as the wall is very thick and the mass of the polymerization reactor is very large. Moreover, the wall vibration is also affected by the way the fluidized-bed is mounted.
Ultrasonic waves and microwaves have been used in the past to measure various material and liquid levels in various containers and vessels. The principle is to put a transducer on the top of the vessel and transmit waves into the vessel. When the waves reach the material surface in the vessel, a part of the waves will be reflected back and received by the same or another transducer on top of the vessel. If the speed of the waves in the gas medium above the material level is known, the time difference between the wave transmitting and receiving can be used to calculate the location of bed level. However, these techniques have never been applied to polymerization reactors because of the special challenges mentioned above and the severe operational conditions of the reactor.
Satoro Watano et al in xe2x80x9cThe Use of Ultrasonic Techniques for the Measurement and Control of Bed Height in Tumbling Fluidized Bed Granulation,xe2x80x9d Advanced Powder Technology 5(2), 119-128 (1994) propose a technique of using ultrasonic waves to measure bed height in a tumbling fluidized bed. An ultrasonic transducer was used on top of the fluidized bed. Although it works successfully in a small size fluidized bed under ambient conditions with low gas velocity and a very small distance between bed level and the ultrasonic transducer (less than 300 mm), it can not be applied to polymerization reactors due to its limitation under high pressure, larger reactor size and the need to constantly calibrate the sound speed in the gas medium on top of the bed level.
The invention involves the use of ultrasonic transducer(s) or microwave transducer(s) installed on the top of the fluidized-bed polymerization reactor. The transducer transmits ultrasonic waves or microwaves as short pulses at predetermined frequencies and intensities and into the reactor freeboard. The waves reflected by the bed surface are received by the same transducer (transceiver) or by a separate wave receiver (transducer). The bed-level can be calculated from the travel time (xe2x80x9ctime of flightxe2x80x9d) of waves reflected by the bed surface. A signal processing unit is used to analyze the receiving signals and determine which of them from each pulse represents the bed level; the data are then processed to achieve a dynamic fluidized bed level.
A wave-speed model developed for high-pressure polymerization-reaction gas is associated with the signal processing unit to supply promptly a factor representing the effect of the instantaneous gas composition on the ultrasonic velocity. The transducer(s) can be installed either non-invasively on the exterior wall of the reactor, or through a port opened at the top of the reactor, preferably so that there is no projection into the reactor. Projection into the reactor, with contemporary technology, should be minimized, but if a way can be found to assure that the projection will not be fouled, or that if it is, the fouling will be obviated by the ultrasonic or microwave readings, such projections are contemplated within our invention.
In addition to being able to detect or measure dynamic bed level in spite of the complicating factors that contribute to it, a measurement system should be able to operate in a wide range of pressure and temperature conditions, it should not be vulnerable to fouling, it should be able to achieve an accurate measurement in spite of an irregular and constantly moving dynamic surface, it should be able to obviate any measurement interference from particles in the freeboard, it should distinguish the bed level from other surfaces in the environment, and it should be able to adjust its readout for continuous changes in gas composition, viscosity, superficial gas velocity, and other characteristics of the suspending medium.
The duration of the pulses may be from 1 millisecond (1 ms) to one second, and the pulse is repeated many times (e.g., 20 to 200 times) with short intervals between consecutive pulses to get an averaged reading of the bed level.
The bed-level detection system of the present invention can achieve prompt bed-level monitoring without the influence of tap plugging and fouling and is not affected by the local defluidization and/or channeling in the reactor. In addition, the environmental and safety problems associated with the nuclear bed-level detector are avoided.
Use of ultrasonic waves or microwaves allows the bed-level measurement to be independent of pressure taps.